The present invention relates generally to semiconductor structures, and more particularly, to semiconductor structures having insulating layers or regions that contain aluminum oxide and to methods for producing such insulating structures.
Insulating materials are employed in various semiconductor structures to provide, for example, isolation between devices formed on a semiconductor substrate and/or to increase the switching speed of such devices. For example, silicon oxide layers are utilized as insulating layers in silicon semiconductor manufacturing. Silicon oxide layers, e.g., SiO2, can be readily produced on semiconductor substrates by utilizing a number of techniques, such as thermal oxidation.
Group III-V semiconductors, such as (AlGa)As, however, do not form adherent, oxide layers as readily as does silicon. The inability to form such native oxide layers inhibits formation of insulating layers that would otherwise provide a simple mechanism for isolation of devices formed on such semiconductors. This, in turn, limits the types of device structures and integrated circuits that are available in Group III-V compound semiconductors.
Thus, a need exists for new and better methods for forming insulating structures in Group III-V semiconductor substrates.
The present invention provides insulating structures of aluminum oxide with substantially stoichiometric composition in a variety of semiconductor circuits. In one aspect, a semiconductor structure of the invention includes a semiconductor substrate having a device formed thereon. An insulating region having aluminum oxide with a substantially stoichiometric composition isolates the device, at least in part, from the remainder of the substrate. The electrical resistance of the insulating region is preferably greater than about 1000 ohm-cm in order to provide effective isolation of the device from selected portions of the substrate.
The term xe2x80x98substantially stoichiometric compositionxe2x80x99, as used herein to describe aluminum oxide produced according to the invention, indicates that the ratio of the oxygen atoms relative to the aluminum atoms is approximately 3/2. In other words, the aluminum oxide can be described by a chemical formula, AlxOy, where x can range from about 0.5 y to about 1.0 y. Preferably, if x is 2, then y is substantially equal to 3.
The semiconductor substrate can include a Group III-V compound having at least one Group III element, and at least one Group V element. The Group III element can be selected to be, for example, Aluminum (Al), Gallium (Ga), or Indium (In); and the Group V element can be selected to be, for example, Nitrogen (N), Phosphorus (P), Arsenic (As), and Antimony (Sb).
The insulating region formed in accord with the teachings of the invention can be a vertical layer providing electrical isolation in a lateral direction, e.g., in a direction parallel to the substrate surface. Alternatively, the insulating region can be a buried horizontal layer having aluminum oxide with a substantially stoichiometric composition. In other applications, the insulating region can be a peripheral layer surrounding an aperture of a semiconductor device, e.g., a vertical cavity surface emitting laser (VECSL).
In a related aspect, the insulating structure can form a continuous aluminum oxide layer, e.g., a film of aluminum oxide. Alternatively, the insulating structure can include a plurality of aluminum oxide precipitates having a concentration that is preferably at least 1% so as to provide sufficient electrical isolation between different portions of the substrate.
In accord with one aspect of the invention, a vertical insulating layer containing aluminum oxide with a substantially stoichiometric composition separates two neighboring devices, formed on the same semiconductor substrate, along a direction parallel to the substrate surface. The high electrical resistance of the insulating layer (e.g., greater than about 1000 ohm-cm) ensures an effective isolation of the neighboring devices in a lateral direction from one another.
In yet another aspect, the invention provides a semiconductor structure having a semiconductor substrate layer, and a waveguide layer formed on the semiconductor substrate. The waveguide layer can have a central portion formed of GaAs that provides a transmission conduit for electromagnetic radiation having a frequency in a selected range. A peripheral portion containing aluminum oxide having a substantially stoichiometric composition surrounds the central portion. The peripheral portion can have an index of refraction, within a frequency range of interest, that differs from the index of refraction of the central portion, e.g., it is lower than the index of refraction of the central portion.
In a related aspect, the invention provides methods for manufacturing semiconductor devices such as those described above. For example, in one embodiment of a method of the invention, an electrically insulating region is formed in a semiconductor substrate by implanting a selected dose of aluminum ions into a portion of the substrate followed by implanting a selected dose of oxygen ions in that portion, or vice versa. The doses of the implanted oxygen and aluminum ions are chosen such that there are substantially three oxygen ions for every two aluminum ions in the implanted region. Subsequently, the substrate is annealed to cause bonding of the aluminum and oxygen ions to produce aluminum oxide with a substantially stoichiometric composition.
The oxygen and aluminum ions can be implanted in the substrate by exposing the substrate to beams of oxygen and aluminum ions having energies in a range of about 0.5 keV to about 2000 keV, and having ion densities in a range of about 5xc3x971015 ions/cm2 to about 1xc3x971018 ions/cm2. More preferably, the energy of the ion beams is approximately 50 to 500 keV.
A mask can be placed on the substrate to allow exposure of a selected portion(s) thereof to the ion beam while protecting the remainder of the substrate from the beams. Such a mask can be formed of a variety of different materials, e.g., SiO2 or Si3N4, and be patterned physically or lithographically.
The annealing process can be performed in different ambient atmospheres and in a temperature range of about 500xc2x0 C. to about 1000xc2x0 C. For example, when the semiconductor substrate is GaAs, the annealing atmosphere can be selected to be an arsine atmosphere. Moreover, the annealing time can be in a range of about 10 seconds to about 48 hours.
If aluminum is one of the atomic constituents of a substrate and is present in sufficient concentration to allow the formation of a continuous aluminum oxide layer or highly concentrated oxide precipitates, the invention can be practiced without implanting aluminum ions by simply implanting sufficient number of oxygen ions into the substrate to form aluminum oxide with a substantially stoichiometric composition.
The insulating structures of the invention can be formed by multiple implantations of aluminum and oxygen ions. For example, vertical insulating layers can be formed by multiple implants of aluminum and oxygen into a vertical portion of the substrate, extending from the surface of the substrate to a selected depth thereof. This can be achieved, for example, by exposing the substrate to ion beams having different energies and ion densities to ensure a substantially uniform distribution of aluminum and oxygen ions in the vertical portion. The energies and the densities of the ion beams can be in a range of about 0.5-2000 keV and 1xc3x971015-1018 ions/cm2, respectively, and are selected such that the ratio of oxygen to aluminum concentration is substantially 3/2. A subsequent annealing process can cause the implanted oxygen and aluminum ions to bond and form aluminum oxide.
The implantation of oxygen ions followed by an annealing step can be sufficient for producing aluminum oxide within a surface region of a substrate if the substrate contains aluminum as one of its constituent atoms.
Another aspect of the invention provides a method for producing an insulating layer surrounding an internal aperture in a vertical cavity surface emitting laser (VCSEL) formed in a semiconductor structure having an active lasing layer and at least one AlGaAs layer above the lasing layer. The method calls for placing a mask on the top surface of the semiconductor structure, which is sized and shaped to produce the internal aperture, followed by implanting a selected dose of oxygen ions in portions of the AlGaAs unobstructed by the mask. A subsequent high temperature annealing step forms aluminum oxide. The oxygen dose is chosen such that the aluminum oxide formed has a substantially stoichiometric composition.
Illustrative embodiments of the invention are described below with reference to the following figures.